RNA interference (RNAi) is an evolutionarily conserved process that functions to inhibit gene expression (Bernstein et al. (2001), Nature 409:363-6; Dykxhoorn et al. (2003) Nat. Rev. Mol. Cell. Biol. 4:457-67). The phenomenon of RNAi was first described in Caenorhabditis elegans, where injection of double-stranded RNA (dsRNA) led to efficient sequence-specific gene silencing of the mRNA that was complementary to the dsRNA (Fire et al. (1998) Nature 391:806-11). RNAi has also been described in plants as a phenomenon called post-transcriptional gene silencing (PTGS), which is likely used as a viral defense mechanism (Jorgensen (1990) Trends Biotechnol. 8:340-4; Brigneti et al. (1998) EMBO J. 17:6739-46; Hamilton & Baulcombe (1999) Science 286:950-2).
An early indication that the molecules that regulate PTGS were short RNAs processed from longer dsRNA was the identification of short 21 to 22 nucleotide dsRNA derived from the longer dsRNA in plants (Hamilton & Baulcombe (1999) Science 286:950-2). This observation was repeated in Drosophila embryo extracts where long dsRNA was found processed into 21-25 nucleotide short RNA by the RNase III type enzyme, Dicer (Elbashir et al. (2001) Nature 411:494-8; Elbashir et al. (2001) EMBO J. 20:6877-88; Elbashir et al. (2001) Genes Dev. 15:188-200). These observations led Elbashir et al. to test if synthetic 21-25 nucleotide synthetic dsRNAs function to specifically inhibit gene expression in Drosophila embryo lysates and mammalian cell culture (Elbashir et al. (2001) Nature 411:494-8; Elbashir et al. (2001) EMBO J. 20:6877-88; Elbashir et al. (2001) Genes Dev. 15:188-200). They demonstrated that small interfering RNAs (siRNAs) had the ability to specifically inhibit gene expression in mammalian cell culture without induction of the interferon response.
These observations led to the development of techniques for the reduction, or elimination, of expression of specific genes in mammalian cell culture, such as plasmid-based systems that generate hairpin siRNAs (Brummelkamp et al. (2002) Science 296:550-3; Paddison et al. (2002) Genes Dev. 16:948-58; Paddison et al. (2002) Proc. Natl. Acad. Sci. U.S.A. 99:1443-8; Paul et al. 2002) Nat. Biotechnol. 20:404-8). siRNA molecules can also be introduced into cells, in vivo, to inhibit the expression of specific proteins (see, e.g., Soutschek, J., et al., Nature 432 (7014):173-178 (2004)).
siRNA molecules have promise both as therapeutic agents for inhibiting the expression of specific proteins, and as targets for drugs that affect the activity of siRNA molecules that function to regulate the expression of proteins involved in a disease state. A first step in developing such therapeutic agents is to measure the amounts of specific siRNA molecules in different cell types within an organism, and thereby construct an “atlas” of siRNA expression within the body. Additionally, it will be useful to measure changes in the amount of specific siRNA molecules in specific cell types in response to a defined stimulus, or in a disease state.
Short RNA molecules are difficult to quantitate. For example, with respect to the use of PCR to amplify and measure the small RNA molecules, most PCR primers are longer than the small RNA molecules, and so it is difficult to design a primer that has significant overlap with a small RNA molecule, and that selectively hybridizes to the small RNA molecule at the temperatures used for primer extension and PCR amplification reactions.